As a circuit board formed by providing a circuit on a surface of an insulating substrate, a technique for forming the circuit by patterning by use of a laser is disclosed (for instance, refer to Patent Citation 1).
In a method of manufacturing the circuit board by such a technique, an insulating substrate 1 is prepared by molding first as shown in FIG. 1(a), then a metal thin film 2 is formed on the entire surface of the insulating substrate 1 by sputtering and the like as shown in FIG. 1(b). Next, a no-circuit forming portion B as an insulating portion for a circuit 3 to be provided on the surface of the insulating substrate 1 is irradiated with a laser L along an outer shape of the circuit 3, thereby removing the metal thin film 2 on the portion irradiated with the laser L as shown in FIG. 1(c). Due to such a removal of the metal thin film 2 by laser irradiation along the outer shape of the circuit 3, it is possible to form a undercoat layer 4 by the metal thin film 2 remained on a circuit forming portion A having the same pattern as the circuit 3. The undercoat layer 4 is isolated from the metal thin film 2 remained on the no-circuit forming portion B. Then, by performing electrolytic copper plating while applying current to the undercoat layer 4, a Cu plating layer 5 is formed on a surface of the undercoat layer 4 as shown in FIG. 1(d). In this point, current is not applied to the metal thin film 2 remained on the no-circuit forming portion B. Thus, the surface of the metal thin film 2 on the no-circuit forming portion B is not subjected to Cu plating. The metal thin film 2 on the no-circuit forming portion B is subsequently removed by soft etching, thereby forming the circuit 3 composed of the Cu plating layer 5 plated on the undercoat layer 4 as shown in FIG. 1(e).
With regard to the circuit 3 formed by the Cu plating layer 5 as described above, while copper and alloy thereof have high electric conductivity, reactive oxide and sulfide may be generated due to corrosion even in normal circumstances, whereby conductivity of electric wirings may be highly detracted. Thus, when the circuit board is used for especially household electronic appliances such as a mobile phone and a mobile digital still camera, a Ni plating layer 6 and a Au plating layer 7 are to be provided on the surface of the Cu plating layer 5 as shown in FIG. 1(f). The Ni plating layer 6 with high adhesion to both Cu and Au is provided between the Cu plating layer 5 and the Au plating layer 7 as a middle layer, in order to protect the Cu plating layer 5 by providing the Au plating layer 7 composed of Au and having good corrosion resistance and contact reliability on the uppermost surface, and prevent Cu and Au from being diffused due to heat load such as a reflow.
Patent Citation 1: Japanese Patent No. 3153682